The present invention relates to a system for recording and playing back audio signal information using digital signal processing and analog signal recording techniques to provide a high quality, low cost professional studio recorder that is easily adapted to handle any number of input signal channels.
In the recording and playing back of audio frequency signals on systems using magnetic tape and other forms of recording media, the reproduction fidelity of the system is limited somewhat by certain physical properties of the recording media. For example, the dynamic recording range of magnetic tape is limited at the high signal amplitude levels by saturation of the magnetic medium which results in distortion of high level signals. At the low end, the tape signal-to-noise ratio causes low signal amplitudes to become buried in the tape hissing sound. In general, the dynamic range of magnetic tape is limited to about 55 decibels.
In the prior art, systems have been proposed for the purpose of expanding the dynamic range of magnetic recording. In one such system, as exemplified by U.S. Pat. No. 3,346,703, an audio frequency analog signal is segregated into separate channels on the basis of amplitude level and the separate signal channels are recorded on the tape on separate recording tracks at comparable dynamic range levels. During playback, one track typically serves as the main channel for normal sound reproduction until the extreme of its dynamic range is reached, at which time provision is made to switch over to the next channel for reproduction of the signal in that channel thus extending the dynamic range of the system. The difficulty with such arrangements is that switching between channels causes distinct unpleasant sound effects and special provisions must be made to assure a smooth transition in volume level when switching occurs. Moreover, when the auxiliary channel is reverted to, for example, the channel in which high sound levels are carried, the low level signals are effectively lost due to the signal attenuation employed to bring the high amplitude signals within the dynamic range of the tape. The result is a loss of clarity and realism in the sound during loud passages.
Digital signal recording techniques have been developed extensively for the purpose of improving dynamic range and playback fidelity, particularly in high quality professional studio recorders for which the present invention is particularly adapted. In such prior art systems, as exemplified by U.S. Pat. Nos. 4,202,018, 4,211,997 and 4,224,642, the audio signal is converted into digital signal form for recording directly in serial word format onto the magnetic tape. With this arrangement, audio signal amplitude is represented on the magnetic tape by the arithmetic value of the recorded digital signal. The original audio signal is then reconstructed from the digital signals during playback by known digital-to-analog conversion techniques. Systems of this type work quite satisfactorily to improve dynamic range and fidelity over straight analog signal recording systems. However, as the referenced patents suggest, digital recording introduces unique problems not associated with analog recording that require complex and expensive signal processing circuits and recording mechanisms to assure correct processing of the digital signals to achieve the desired results. For example, the loss of a digital bit from any relatively significant bit position can be catastrophic from the standpoint of sound reproduction in terms of errors in amplitude of the reproduced signal. The result is a snapping or popping dound that degrades the quality of the reproduced program material. Moreover, splicing of digitally recorded tapes is not a convenient process since timing can be lost at an incorrectly spliced tape junction with consequent adverse effects on the reproduced sound.
Although magnetic tape is widely used for recording playback systems and particularly for multi-channel studio recorders, other recording media are well known and exhibit similar dynamic range problems especially with respect to signal-to-noise ratio. Examples are magnetic discs, laser beam recording, thermal deformation, video disc, as well as others, and it will be appreciated from the description that follows that the system and method of the present invention is useful with these other media as well as magnetic tape.
It is, therefore, an object of the present invention to provide a record/playback system useful with a variety of recording media which overcomes the aforementioned problems associated with prior art audio record/playback systems.
It is a further object of the invention to provide a record/playback system that does not require complex and expensive mechanisms for digital recording on magnetic tape and other recording media and which is capable of being used with conventional forms of multiple track analog signal recording mechanisms.
It is a further object of the invention to provide a hybrid analog-digital signal recording/playback system which improves the dynamic recording range of the system without the disadvantages associated with pure digital signal recording systems.
It is a still further object of the invention to provide an audio recording/playback system suitable for use with magnetic tape and other recording media that provides improved signal-to-noise ratio performance over prior art systems employing analog signal recording techniques on the recording media.
It is yet another object of the invention to provide a low cost, high quality profession studio recorder system having multiple signal recording channels.
These and other objects and advantages of the invention will be apparent from the detailed description of the invention when taken in connection with the various figures of the drawing.